Solids Treatment Before Metals Removal

Depending on the characteristics of the CMP waste stream, the oxidizer, and metals treatment technologies that are selected, removal of the solids from the waste stream prior to metals removal may be required. For example, if a reverse osmosis array or electrodeionization is used to remove the trace metals, then the suspended solids must be removed from the feed, otherwise the equipment will clog and fail catastrophically. To remove the suspended solids, 

FIGURE 19.6 A schematic of a CMP waste treatment system with solids concentration and removal as the primary unit operation. Although this is a feasible method for treating CMP wastewater, it is the most expensive option because of the cost of transporting hazardous waste. 

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Water Treatmeat and Supply. Before water is consumed, it has to be collected first from either underground or above-ground sources. Therefore, source control is one of the most important tasks of water supply. Except for a few municipalities where the source water derived from deep aquifers, source water has to be treated to remove contaminants such as pathogenic bacteria, heavy metals, and pesticide residues. The process of treatment involves the removal of suspended solids and the use of chemicals or ultraviolet (UV) radiation to disinfect unwanted organisms so that the effluent water satisfies quality requirements dictated by the Federal Safe Drinking Water Act. For water used by industrial plants such as paper mills or nuclear power plants, special treatment is needed and its discharge is regulated. 

Common pollutants in a titanium dioxide plant include heavy metals, titanium dioxide, sulfur trioxide, sulfur dioxide, sodium sulfate, sulfuric acid, and unreacted iron. Most of the metals are removed by alkaline precipitation as metallic hydroxides, carbonates, and sulfides. The resulting solution is subjected to flotation, settling, filtration, and centrifugation to treat the wastewater to acceptable standards. In the sulfate process, the wastewater is sent to the treatment pond, where most of the heavy metals are precipitated. The precipitate is washed and filtered to produce pure gypsum crystals. All other streams of wastewater are treated in similar ponds with calcium sulfate before being neutralized with calcium carbonate in a reactor. The effluent from the reactor is sent to clarifiers and the solid in the underflow is filtered and concentrated. The clarifier overflow is mixed with other process wastewaters and is then neutralized before discharge. 

Oxides, hydroxides, free metals, and simple alloys If a solid substance is found to contain no anions, it may be an oxide, or hydroxide, or a metal or a mixture of metals, or an alloy. Metals and alloys have certain characteristic physical properties many metals evolve hydrogen on treatment with dilute acids. As a rule, nitric acid must be employed as solvent, and it will then be necessary to remove the excess nitric acid (as already described above) before proceeding to the Group analysis. 

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